Fix Howl UI layout for some version/combination of WM/compositor, etc.

As seen on ElementaryOS, there was a problem that manifested itself as a very
wide margin outside of the actual Howl window. Issue
200 has more information.

FreeBSD build fixes

Thanks to maxc01 Howl now builds cleanly out of the
box on FreeBSD without the need for any patches (still requires gmake though).

Themable scrollbars

Scrollbars can now be themed as well (requires newer versions of Gtk). While
this may not sound like a show stopper in itself, the lack of styling became a
real problem when the default scrollbar color blended in to well with a theme,
in effect making it invisible.